1. Field of the Invention
The present invention relates to the technique of frequency modulated continuous wave radar systems; it relates more precisely to radar equipment for providing, over a wide distance range, a precise measurement of the relative distance away from a reflecting object; it also relates to a radioelectric altimetric probe.
2. Description of the Prior Art
The technique of frequency modulated continuous wave radars, or FM/CW radars, is widely known and described in numerous specialized works, This technique is almost numerous specialized works. This technique is almost universally used in aircraft radio altimeters and similar radar equipment. The main advantages of FM/CW radar reside in its relative simplicity of construction and in its capability of providing a precise measurement of the relative distance away from a reflecting object situated within its detection range. On the other hand, these FM/CW radars of the Prior Art present constructional restrictions and technical limitations which result from the requirement of having two separate antennas: a transmitting antenna and a receiving antenna, having generally radioelectric characteristics and identical mechanical structure. In fact, to ensure a sufficient radioelectric decoupling factor of the two antennas, so as to eliminate interferance phenomena between the transmitter and the receiver, the antennas must be physically remote from each other; certain radioelectric and mechanical problems arise therefore because of the fact that these antennas cannot be integrated in the equipment, but must be fixed to the wall of the carrier vehicle and interconnected with the equipment through transmission lines whose transmission delay must be measured; these transmission lines introduce propagation losses and multiple reflections of the transmitted signals. Moreover, when the relative distance from the object to be detected is extremely reduced, multiple path phenomena may appear between the object and the wall of the carrier vehicle, for example, when the object to be detected is formed by the ground or an obstacle of large extent; these multiple paths cause erroneous distance measurements. It may also be noted that, when the distance separating the antennas is no longer negligible with respect to the distance away from the object to be detected, the geometry of the antennae system becomes imperfect. In addition, among the mechanical problems raised by fixing the two antennas on the wall of the carrier vehicle, may be mentioned the requirement of reserving inside the vehicle housings for the antennas, passages for the transmission lines and accesses to the different connectors for installing and dismantling the different connections; finally, the electric continuity of the wall of the carrier vehicle must be preserved, which requires certain precautions to be taken during construction of the wall of the vehicle.
In FM/CW radar equipment, if the transmitted signal and the signal reflected by an object are duplexed on a common antenna, there is produced a composite coupling signal between the transmitter and the receiver; this composite coupling signal results mainly from a partial reflection at the level of the antenna which has a standing wave rate of a finite and not zero magnitude, and from a leak signal introduced by the duplexer circuit whose coefficient of directivity is logically limited. It follows then that this radio frequency composite coupling signal, after demodulation in the input mixer of the receiver, creates at the audio frequency amplifier of the receiver interference signals which are partially situated in the effective pass band of the echo signal; the result is a limitation in the sensitivity for detecting a close object.
The aim of the present invention is to overcome the above mentioned limitations of the FM/CW radars of the Prior Art.